Impact mills



Dec- 26, 1961 R.`F. BOURNE ET AL 3,014,669

IMPACT MILLS Filed Aug. 22, 1960 United States Patent Otice 3,014,659 Patented Dec. 26, 1961 3,014,669 IMPACT MILLSV Ronald F. Bourne, Wynnes Oak Road, Atholl, and Donald G. Cheyne, 120 Lancaster Ave., Craighall Park, both of Johannesburg, Transvaal, Union of South Africa Filed Aug. 22, 1960, Ser. No. 51,048 Claims priority,application Union of South Africa Sept. 10, 1959 3 Claims. (Cl. 241-188) This invention relates -to i-mpact mills.

Air-swept mills in which the raw material is ground by impact and/or attrition and in which oversize particles are returned to the milling zone by a rotary separator device whichpasses the fines but rejects the oversize particles centrifugally, are known.

The present invention relates to mills of the abovementioned general type and its principal object is to provide new or improved milling apparatus.

The invention is also concerned with replaceable parts for the new or improved mill.

According -to the invention a mill of the kind referred to comprises a pair of spaced shrouds extending in a radial direction wi-th respect to the axis of rotation of the mill, said. shrouds beingconvergent in a radially outwards direction; a rotor having a plurality of radial blades which extend completely across the space between the shrouds to which latter they are secured; a casing enclosing the peripheral zones of the blades and shrouds and mounted in spaced relationship thereto to provide a passage through which air may be admitted to the mill between the casing and the shroud-s thereafter to pass radially inwards towards the rotational axis; a port through which raw material may be fed to the mill at the outer extremities of the blades; and an axial duct through which milled material together with air may pass out of the mill.

Further, according to the invention there is a plurality of radially extending in-terrupter plates within the casing.

Preferably the plates are mounted in staggered relation- Vship with respect to each other.

The invention also provides an arrangement wherein "the tips of the blades are convex and the tips of the plates are concave.

Also according to the invention a rotor for use in an impact mill of the kind referred to includes a plurality of radial blades and a pair of spaced shrouds extending in "of FIGURE 1, and

FIGURE 3 is an inside, elevation of the liner with parts broken away showing the location of the interrupter plates.

Referring to the drawings, a hub 1 is mounted on a shaft 2. An annulus 3 is carried on the outer end of the hub 1 and a shroud 4 in the form of an annulus is attached to the annulus '3. Y

A second annular shroud 5 is supported in spaced relationship to the shroud 4 by radial blades 6 which extend between the two shrouds 'and are secured thereto.

The inner surfacesof the two shrouds 4 and 5 are convergent in a radially outwards direction so that an annular space is defined' between themwhose volume at differing radial zones remains substantially constant or progressively increases towards the axis of rotation of the rotor constituted by the hub 1, the shrouds 4 and 5, and the blades 6. The direction of rotation of the rotor is shown by arrows in FIGURE l.

7 is an axial suction duct which is stationary and which supports an annular seal 8 located between an abutmen ring 9 and the inner portion 10 of the shroud 5. l

As Shown in FIG. 1, four blades 6 may conveniently be employed.

A casing 11 of substantially U-shaped cross-section surrounds the periphery of the shroud and blade structure carried by the hub 1 and is mounted in spaced relationship to such structure.

A liner 12 of substantially U-shaped cross-section with the arms ofthe U diverging from one another as shown, is located within the casing 11.

Interrupter plates 13 extend radially inwards from the liner 12 and such plates are staggered with respect to one another. FIGURE 3 gives a clear picture of the staggered relationship of the plates to each other.

14 is a tangential inlet through which raw material may be introduced into the mill.

As shown in FIG. 2, the milling tips of the blades 6 are convex and the adjacent inner edges of the interrupter plates 13 are of complementary concave shape.

In use, raw material to be ground is introduced into vthe mill through the inlet 14- and air is drawn through the mill between the casing 11 and the shrouds 4 and 5 by a fan (not shown) associated with the suction duct 7. The lines are removed from the mill via the duct 7 and oversize particles are thrown back into the milling zone by centrifugal force.

Amongstvthe advantages possessed by a grinding mill according to the invention the following may be mentioned, viz:

(l) The need for accurate machining is reduced considerably. This is evident from a consideration of the described example of the invention.

(2) The rotor blades serve the dual purpose of fracturing the raw material and of throwing oversize particles back into the milling Zone. This permits a steady flow of lines from the milling zone through the ricochet zone (A--A in FIG. 2) and past the classification line or li-ne of no return (B-B in FIG. 2), without disturbance.

(3) The shape of the shrouds is such that the annular area between them in successive radial zones may be kept constant or it may be allowed to increase. This allows the air, which is moving radially inwards, to retain a constant or decreasing velocity, whereas parallel sided shrouds would result i-n an increasing air velocity.

This constant or decreasing centripetal air velocity reduces the rapid cumulative effect on the particles of materials that obtain with parallel sided shrouds and which is due to reduced centrifugal force together with a rap-idly increasing centripetal air velocity. This, in

turn, leads. to better classification with fewer oversize' particles in the product. Oversize particles which have accidentally ricocheted over rthe classilicaton line have a better chance of moving radially outwards again back to the milling zone at the .tips of the blades.

(4) The converging shrouds assist in the mechanical retention ofthe blades against centrifugal force which is considerable owing to the highspeed of rotation.

(5) The divergent arms of the liner allow:

(i) An easy deection of the burst of material after impact with the interrupter plate across both incoming air streams, first from one side and then from the next and so on. This results in the essential washing of the material and in the extraction (down the blades) of the lines as they are made.`

The result i=s probably a type of helical ow of the oversize particles, rst on one side of the blade and then on the other, `until the particles are line enough to be washed down the blade by the air against the centrifugal force.

It should be noted, incidentally, that if the lines are not extracted as Soon as the desired degree of lineness has been reached, unnecessary power is absorbed in dragging them around.

(ii) The interrupter plates to be deepest in the centre where the heavy or oversize particles concentrate as the result of the air ow effect, and the interrupter plates to be shallow at the sides thereby permitting the air to rotate more easily as it enters between the casing and the shrouds.

(iii) Deflection of the rotating and outwardly moving air towards the vertical centre of the casing, thereby allowing it to penetrate between each interrupter plate to cause further washing of the material.

(6) The staggered interruptor plates, which do not extend across the full width of the casing, allow the rotating, turbulent mass of material and air to flow round more freely with the result that less power is absorbed.

v(7) Because the material rebounds violently from the liner back to the blades after each burst or impact, and because milling is effected by repeated impact and not by grinding as such, the radial distance between the blade tips and the interrupter plates is not at all critical and it caribe comparatively large. No great accuracy is therefore called for and various materials, such as fibrous materials, which are difficult to mill can be `fed and processed. Also, the feed size can be comparatively large.

(8) The convex blade tips and the corresponding concave edges of the interrupter plates permit impact of the material to accur at a point on the plate which is further out radially than would be the case if straight blade tips and interrupter plates were used. It is logical to suppose that this allows the material to rebound from the liners `from points further out rather than right on top ofthe incoming air streams.

Again, whereas in the mill illustrated, the raw material enters tangentially, it can also be fed in with the air stream through a hopper. Tangential feed is, however,

preferred since not only does it not have such a limiting iniluence on the size of particles introduced, but it causes a break in the rotation of the general mass of material and air which has a loosening effect.

Many more examples of the invention exist each dillering from the other in detail but in no way departing from the principles set out in the claims.

We claim:

l. An impact millcomprising a pair of spaced shrouds extending in a radial directionl with respect to the axis of rotation of the mill, said shrouds being convergent in a radially outwards direction, a rotor having a plurality of radial blades which extend completely across the space between said shrouds to which latter they are secured; a casing enclosing the peripheral zones of said radial blades and shrouds and being mounted in spaced relationship thereto to provide a passage through which air is admitted to the mill Abetween said casing and said shrouds thereafter to pass radially inwards towards the rotational axis, a port through which raw material is fed to the mill disposed at the outer extremities of said radial blades, and an axial duct through which milled material together with air passes out of the mill.

2. The mill of claim 1, including a plurality of radially extending interrupter plates mounted within the casing, the tips of said radial blades being convex and the tips of said interrupter plates being concave.

3. For use in an impact mill, a rotor having a plurality of radial blades and a pair of spaced shrouds extending in a radial direction with respect to the axis of rotation of the mill, said shrouds being convergent in a radially outwards direction and said radial blades extending completely across the space between said shrouds to which latter they are secured.

References Cited in the file of this patent UNITED STATES PATENTS 990,633 Campbell Apr. 25, 191l 2,294,921 Lykken Sept. 8, 1942 2,414,361 Cowles Ian. 14, 1947 2,839,251 Hippert .lune 17, 1958 FOREGN PATENTS 203,215 Australia .lune 23, 1955 

